Shell or mv eats a period?

[bob prohaska]

During a recent update of freebsd-arm on an rpi2 I happened to type
root at ns1:/boot/msdos # ls
bootcode.bin	fixup_cd.dat	readme		start_cd.elf	ubldr
config.txt	fixup_x.dat	rpi2.dtb	start_x.elf
fixup.dat	licence.bro	start.elf	u-boot.bin

To prepare for upgrading from r494499, I then typed

root at ns1:/boot/msdos # mv rpi2.dtb rpi2.dtb.old

root at ns1:/boot/msdos # ls
bootcode.bin	fixup_cd.dat	readme		start_cd.elf	ubldr
config.txt	fixup_x.dat	rpi2dtb.old	start_x.elf
fixup.dat	licence.bro	start.elf	u-boot.bin

Note that the renamed file has lost the first dot. Next, the new file
was copied into position:

root at ns1:/boot/msdos # cp /boot/dtb/rpi2.dtb /boot/msdos
root at ns1:/boot/msdos # ls
bootcode.bin	fixup_cd.dat	readme		start.elf	u-boot.bin
config.txt	fixup_x.dat	rpi2.dtb	start_cd.elf	ubldr
fixup.dat	licence.bro	rpi2dtb.old	start_x.elf
root at ns1:/boot/msdos #

It's happened twice so far, so it's unlikely to be a wetware error 8-)
Can anybody tell me what happened?

thanks for reading,

bob prohaska